Composite materials are widely used in the fabrication of products ranging from tennis racquets to advanced aerospace structures. One type of composite material used in aerospace structures is available in the form of a sheet consisting of continuous parallel reinforcing fibers embedded within an organic matrix material. Typically, the matrix material is an adhesive system such as an epoxy resin. The sheet consisting of reinforcing fibers and matrix material is known as "prepreg."
Prepreg is commonly provided from a manufacturer as a continuous sheet wound on a roll or spool, with the fibers extending longitudinally around the spool. The prepreg can be purchased in widths ranging from centimeters to meters. Typically, the wider prepreg spools are used in hand layup procedures in which individual layers or plies of prepreg are manually placed on top of each other by a worker. The narrower spools or tapes are typically used in automated tape-laying machines or winding machines.
Automated tape-laying machines use a continuous spool of composite prepreg and a robotic arm to apply individual plies of prepreg to a pan or substrate in the desired fiber orientations. The tape-laying machines are capable of starting an individual ply at the location desired, laying up the ply over the distance desired, and then ending or cutting the ply at the location desired. Many tape-laying machines use a robotic ann that terminates with a round application roller. As the robotic arm moves, the roller brings the prepreg into contact with the part and places a force on the prepreg in the direction of the surface of the part. The force causes the prepreg to adhere to the underlying substrate or part. As the robotic arm moves forward, the tack of the prepreg maintains the individual plies in position. If the tack of the prepreg is insufficient, the prepreg does not adhere to the underlying substrate.
The tape-laying machine's ability to properly place the prepreg tape, and thus the resulting part quality, is greatly influenced by the stickiness or tack of the prepreg used on the machine. Prepreg tape that has insufficient tack can result in the prepreg shifting during layup or lifting or sliding out of the proper position after being laid up. This undesirable movement can prevent fabrication of the part altogether, require additional manual adjustment, or can result in parts that have voids or gaps between individual prepreg tapes or which are not within the desired dimensional or structural tolerances. Similarly, prepreg with too high a degree of tack can also cause improper placement of the prepreg tape or can prevent repositioning of the prepreg after improper placement.
In order to work properly in a tape-laying machine, the prepreg must have the proper tack to prevent shifting, lifting, or sliding of the laid-down prepreg over the typical laydown rate profile of the tape-laying machine. Tape-laying machines generally begin a laydown pass with the application roller stopped and in contact with the composite part. The tape-laying machine then accelerates to a predetermined laydown speed, which it maintains over the majority of each laydown pass. The tape-laying machine then decelerates near the end of each laydown pass in order to increase compaction, and thus prepreg adherence, prior to cutting the prepreg and completing the laydown pass.
To work properly, the laid down prepreg must adhere to the underlying substrate during the acceleration portion of the laydown pass, during the constant-speed portion of the laydown pass and during the deceleration portion and subsequent cut-off portion of the laydown pass. Failure of the prepreg to remain adhered during any part of the laydown pass could create flaws in the final part or require the prepreg to be manually compacted in areas where the prepreg did not adhere properly.
A number of variables influence the tack of the prepreg and its adherence to an underlying substrate. Such variables include the matrix material used in the prepreg, the temperature and humidity at which the prepreg is laid up, the rate at which the prepreg is laid up, the force used to place the prepreg, the age of the prepreg, the durometer of the material that forms the application roller, the diameter of the application roller, and the amount of tithe the prepreg has been exposed to the air prior to use. A number of these variables are directly influenced by the material properties of the prepreg itself, and can differ with each batch of prepreg received from the manufacturer. Each batch of prepreg often has a different value of tack, due to minor changes in the reinforcing fibers, matrix, or prepregging procedures.
A limited range of tack differences can be accounted for by adjusting the tape-laying machine, in order to control the force used to layup the prepreg, the rate at which the prepreg is laid up, the durometer of the material from which the application roller is formed, and diameter of the application roller. Adjusting the diameter and the durometer of the material from which the roller is formed changes the way the force is applied to the prepreg, thus influencing the prepreg's adherence to a substrate. However, if the tack of the prepreg is not within a suitable range, adjusting the tape-laying machine cannot compensate for the poor quality prepreg. Therefore, prepreg that has an improper amount of tack should be rejected upon receiving the prepreg from the manufacturer.
In the past, there has not been a standardized procedure to monitor the tack of composite prepreg prior to using the prepreg in a tape-laying machine. Therefore, there has been no standard measurement available that could be used as a quality control variable in accepting or rejecting different batches of prepreg. In the past, each roll of prepreg would be placed in the tape-laying machine before it could be determined whether or not the prepreg was acceptable. This resulted in a loss of time and money in setting up the tape-laying machine and performing trial runs prior to determining whether the prepreg was acceptable.